1. Polymorphic nanocrystalline
metal oxides
Thermodynamics And Applications
Shantanu Sood
Department of Materials science and engineering

2. Layout of the presentation
• Nanocrystalline Metal oxides
Elaborate the importance of nanoscale for polymorphic
metal oxides
• Thermodynamics of polymorphic transitions
Explanation of a thermodynamic model to explain the
differing transitions due to nanoscale
• Applications
One material many structures, differing behavior

3. Ceramic Materials analysis
• Binary metal oxides are some of the most useful
materials and the modifications serve as the basis of
our civilization.
• Nanocrystalline metal oxides are of current research
interest.
• Synthesis Techniques:
– Sol-Gel
– Electrospinning
– Flame Spray Pyrolysis
• Characterization Techniques:
– XRD
– Electron Microscopy
– Differential Scanning Calorimetry

4. Polymorphs due to Phase transition
• Various polymorphs of metal oxides occur due
to phase transitions.
• In bulk size(micrometer or higher grain
size), temperature and pressure are the factors
that affect phase transition.
• In nano size(100nm or less), temperature and
particle size are the two factors that contribute
to phase transition
• It is observed that there is a lowering of external
energy required for phase transition at nano
scale, this helps lower the temperature and
pressure conditions

5. Example
Sood(2012) Bulk state Nano state
Particle Size Micron-size 8nm
Transformation 1473K 873K
Temperature
Ref. [1],[2]
Tetragonal(I36) Tetragonal(I41/amd)
1. P. I. Gouma and M. J. Mills, "Anatase to Rutile Transformation in Titania Powders", J. Am. Ceram. Soc., 84 [3], pp. 619-622, 2001.
2. M. R. Ranade, A. Navrotsky, H. Z. Zhang, J. F. Banfield, S. H. Elder, A. Zaban, P. H. Borse, S. K. Kulkarni, G. S. Doran , and H. J. Whitfield. National Acad
Sci., vol. 99 no. Suppl 2, 2002, 6476-6481, DOI: /10.1073/pnas.251534898PNAS

6. Other Examples of polymorphs from
literature – Bulk and Nano conditions
Phase Bulk Nano Ref.
γ-Fe2O3 to α- 933K 5-25nm 563-673K [1],[2]
Fe2O3
Monoclinic to 1143K 10nm Room T [3]
Tetragonal ZrO2
α-WO3 to ε- 220K -- Room T [4]
WO3
γ-Al2O3 to α- 773K 3.2nm Room T [5],[6]
Al2O3
1. Fu Su Yen, Wei Chien Chen, Janne Min Yang, and Chen Tsung Hong. Nano Letters, Vol. 2, No. 3, 2002, 245-252, DOI: /10.1021/nl010089m
2. Ozden Ozdemir and Subir K. Banerjee, Geophysics research letters, Vol. 11, No. 3, 1983, Pages 161-164, DOI: /10.1029/GL011i003p00161
3. R. C. Garvie, M. F. Goss. J. Mater. Sc. 21, 1986, pp 1253-1257, DOI: /10.1007/BF00553259
4. L. Wang, A. Teleki, S. E. Pratsinis, and P. I. Gouma. Chem. Mater. , 20, 2008, 4794–4796, DOI: /10.1021/cm800761e
5. Shuxue Zhou, Markus Antonietti, and Markus Niederberger. Small 3(5), 763(2007).
6. .J. M. McHale, A. Navrotsky, A. J. Perrotta, J. Phys. Chem. B, 101 (4), 1997, pp 603–613, DOI: /10.1021/jp9627584

7. Thermodynamic model for explanation
Bulk Nano
For equal mass in grams of material, In nanometer
Bulk volume = Nano volume dimensions, grain size is so
Nano number of grains >>> small that most atoms are
Bulk number of grains surface
Total Surface area = Atoms exerting very high
(number of grains).(4).(3.14).(r)2 pressure.
Surface atoms have high
This leads to a very high charge due to unfilled energy
Surface area to volume ratio. bands and broken bonds.
Expression for Bulk state phase
transformation.
This causes an exponential increase This cause internal
in surface energy pressure.

8. Surface Area effect[1] Internal Pressure effect[1],[2]
ΔP for water drops of different radii
Droplet 1 mm 0.1mm 1μm 10nm
radius
ΔP (atm) 0.0014 0.0144 1.436 143.6
From thermodynamics we know that at the point of equilibrium, free energy is zero,
thus, solving for critical particle size, ‘r’,
1. Jiang, Q. Yang, C. C. Current Nanoscience Vol. 4 Issue 2, May 2008, , pp179-200, DOI: /10.2174/157341308784340949
2. Sheryl H. Ehrman, Journal of Colloid and Interface Science. Volume 213, Issue 1, May 1999, Pages 258–261, DOI: /10.1006/jcis.1999.6105

9. Lowering of activation barrier due to
particle size
• In bulk, external pressure
is required to overcome
the barrier for phase
transition.
• But at nano size, the
internal pressure and
surface effects contribute
and lower the barrier
making available the high
pressure phases at
ambient conditions.
• Thus increasing the
spectrum of phases that
are available for each
material

10. Applications

11. Gas Sensing
β-MoO3 on NH3 gas.[3] ε-WO3 on acetone gas.[2] Anatase TiO2 on CO gas.[1]
Orthorhombic Structure Monoclinic Structure Tetragonal Structure
Grain Size = 50nm Grain Size = 20nm Grain Size ~ 13.2nm
Temperature = above 425K Temperature = Room Temp. Temperature = 773K
1. Ana M. Ruiz, Albert Cornet, Kengo Shimanoe, Joan R. Morante, Noboru Yamazoe. Sensors and Actuators B: Chemical. Vol 108, Iss 1-2, July 2005, Pages 34-40, DOI:
/10.1016/j.snb.2004.09.045
2. L. Wang, A. Teleki, S. E. Pratsinis, and P. I. Gouma. Chem. Mater. , 20, 2008, 4794–4796, DOI: /10.1021/cm800761e
3. Arun K. Prasad’s. Phd thesis, Stony brook university, May 2005.

12. Catalysis – Solid Oxide Fuels Cells
SOFCs are an oxygen ion conducting electrolyte through which the oxide ions migrate from the
environment electrode (cathode) side to the fuel electrode (anode) side reacting with the fuel
(H2, CO, etc.) thereby generating electrical voltage.
Mesopore size distribution and nanocrystalline channel walls lead to improvements[1] in:
• fuel mass transport,
• oxide ion mobility,
• electronic conductivity, and
• charge transfer
Cubic Zirconia[2], as a Catalyst
• Yttrium stabilized Nanocrystalline Cubic Zirconia
• Benefits like, uniform intergranular pore size and greater
oxide ion conductivity due to yttrium stabilization Bloom Energy
Polymorphs of Bismuth Oxide[3], as catalyst
• Bismuth oxide based systems have higher ion conductivity than Zirconia based systems.
α-Bi2O3 β-Bi2O3 γ-Bi2O3 δ-Bi2O3 Ref
Ion Conductivities(Scm-1) 3X10-4 2X10-3 5X10-3 1 [3]
1. Marc Mamak, Neil Coombs, and Geoffrey Ozin. J. Am. Chem. Soc., 122 (37), 2000, pp 8932–8939, DOI: /10.1021/ja0013677
2. S.C Singhal. Solid State Ionics. Vol 135, Iss 1–4, November 2000, Pages 305–313, DOI: /10.1016/S0167-2738(00)00452-5
3. Laarif, A. and Theobald, F. Solid State Ionics, 21, 1986, 183-193, DOI: /10.1016/0167-2738(86)90071-8

13. Electrochemical Cells and Batteries
• Ions like H+, Li+, Na+, K+ etc intercalate in to the lattice of polymorphic metal
oxides
• Some structures have a better intercalation capacity and charge discharge
capacities than others making them better for charge storage applications
Example
• Hexagonal MoO3 show better charge storage capacity
than orthorhombic MoO3[1],[2]
Li Intercalation Discharge
Capacity capacity
Orthorhombic MoO3 1.5Li/MoO3 300mAh/g
Hexagonal MoO3 2.2Li/MoO3 400mAh/g
• Similarly, hexagonal WO3 also readily form Tungsten
oxide bronze(MxWO3), and has better intercalation Sood(2012)
capacity than orthorhombic WO3[3]
1. Jimei Song, Xiong Wang, Xiaomin Ni, Huagui Zheng, Zude Zhang, Mingrong Ji, Tao Shen, Xingwei Wang. Materials Research Bulletin. Vol 40, Iss 10, October
2005, Pages 1751–1756, DOI: /10.1016/j.materresbull.2005.05.007
2. S.H. Lee, M.J. Seong, C.E. Tracy, A. Mascarenhas, J.R. Pitts, S.K. Deb. Solid State Ionics, 147, 2002, p. 129, DOI: /10.1016/S0167-2738(01)01035-9
3. K.P. Reis, A. Ramanan, M.S. Whittingham, J. Solid State Chem. 96, 1992, pp 31-47, DOI: /10.1016/S0022-4596(05)80294-4

14. Conclusion
• Nano scale makes available polymorphs of metal
oxides that were hitherto unavailable due to
conditions of high pressure and temperature
involved
• The internal pressure and surface energy due to
nano dimensions helps compensate for high
pressure needed externally in bulk state
• Some polymorphs which have better properties
can now be used in applications like as
sensing, catalysis etc, as no high pressure
synthesis is required

15. Thank You